High Strain Hardening Research Articles

Dry machining of Inconel 718 super alloys using uncoated tool: Experimental and numerical analysis

Aerospace super alloy Inconel 718 is difficult to process through conventional machining operation. Alloys with high strength at high temperatures and high strain hardening, high chemical affinity (towards tool material, and Co binder) etc. impose adverse effects towards smooth machining. Poor thermal conductivity of Inconel 718 promotes excessive temperature rise at the chip-tool interface which causes rapid tool wear, and degraded surface integrity of the end product. Adequate understanding of machining process phenomena along with precise control of machining parameters may yield satisfactory result. Trial, and error experimentation is indeed uneconomical; hence, in the present reporting, Finite Element (FE) based numerical simulation is attempted to model machining responses in the extent of cutting force, tool-tip temperature, depth of flank wear progression, chip-tool contact length, and finally, chip reduction coefficient. Simulation results are verified through experimental tests. Simulation results are found in good agreement with experimental results. Therefore, simulation results can reliably be used as an alternative instead of actual experimental effort.

The formability of twinning—Induced plasticity steels predicted on the base of Marciniak-Kuczynski theory

Abstract The purpose of this work is to predict and analyze the formability of twinning – induced plasticity steels through the Marciniak-Kuczynski (MK) theory with emphasis on the solutions for improving the prediction results. The selected constitutive equations involve the Yld00-2d of Barlat et al. (2003) plane stress yield function, the Swift strain–hardening power law and the dislocation density based constitutive model proposed by Kim et al. (2013), taking into account the dislocation glide, twinning and dynamic strain aging. Three types of high manganese content TWIP steels sheet were selected. To understand the formability of the TWIP steel and the factors influencing it, a sensitive study on the effect of the mechanical properties of the TWIP steel on the MK theory concept and the predicted forming limits is performed. Using the dislocation density based microstructural model, the deformation twinning effect and the contribution of dynamic strain aging to the FLDs of TWIP steel is analyzed. The relevant influence of the sharpness of the yield surface in the biaxial stretching region in the prediction of FLDs of TWIP steels is highlighted. The extended MK model can be adapted to predict the forming limits of the TWIP steels by using an unusual high initial geometrical defect imposed by their high strain hardening. In this way it was showed that the MK theory cannot be applied for predicting the forming limits of TWIP steels unless by applying imperfection factors that are not physically reasonable. Therefore, new failure models are required for TWIP steel.

Improving the high-temperature ductility of Al composites by tailoring the nanoparticle network

Al-matrix composites with networked ceramic nanoparticles have been reported to possess high thermal stability, but their low ductility at high temperatures limits their wide applications. In this study, rotary swaging (RS) was used to modify the network of AlNp nanoparticles for optimizing the high temperature tensile properties of Al matrix composites. The RS treatment was found to disperse the network structure of AlNp particles to some extent and to refine the grains of Al matrix, which resulted in high strain hardening and strain rate sensitivity. This not only improved the thermal stability but also increased the uniform elongation from 0.99% to 2.1% at 350 °C. This work provides a new pathway for tailoring the nanoparticle architecture to obtain a desirable strength-ductility synergy for metal matrix composites.

Development of high strain-hardening lightweight engineered cementitious composites: Design and performance

This paper presents a newly developed high strain-hardening, lightweight, engineered cementitious composites, using fly ash cenospheres as the filler. That has tensile strain capacity superior to that found in other lightweight concrete materials. Test results indicate: (1) The pseudo strain-hardening indices of the new engineered cementitious composite are significantly larger than the minimum requirement specified in the literature which guarantees an ultra-high ductility. (2) Even though the increase in fly ash cenospheres content reduces the tensile and flexural strength of the high strain-hardening, lightweight, engineered cementitious composite, it has little effect on its ductility; moreover, fly ash cenospheres can significantly reduce tensile stress fluctuations and crack width. (3) A calcium silicate hydrate gel forms tightly around the fly ash cenospheres due to the pozzolanic effect of fly ash cenospheres, which increases the matrix strength and provides low density matrix to maintain high tensile, flexural and compressive properties. (4) The polyethylene (PE) fiber underwent a large extrusion deformation during extraction leaving behind a large amount of broken mortar and fly ash cenospheres fragments on the fiber surface, which gave the new composite a super high tensile strain capacity.

Beta-type Ti-Nb-Zr-Cr alloys with large plasticity and significant strain hardening

A series of Ti-25Nb-8Zr-xCr (x = 0, 2, 4, 6, 8 wt%) alloys were designed based on DV-Xα cluster method and e/a¯-Δr¯ diagram with an anticipation to obtain high plasticity and significant strain hardening. The designed alloys were produced through cold crucible levitation melting technique in order to effectively investigate their microstructures and mechanical properties. The addition of Cr significantly enhances the β stability in the microstructures of the Ti-25Nb-8Zr-xCr alloys. Both yield strength and hardness of the studied alloys increase due to the effect of solid-solution strengthening. By constrast, the plasticity, maximum strength and strain hardening rate are influenced by the β stability as well as the distinct deformation mechanisms. None of the alloys comprising Cr fail up to 100 kN (the load capacity used) and all show impressive plasticity (~75%) and superior maximum compressive strength (~4.5 GPa) at 100 kN. Moreover, the deformation bands, which are found around the hardness indentations, are analyzed for all the investigated alloys. The fracture behaviors of the Ti-25Nb-8Zr-xCr alloys are also studied to observe the characteristics related to crack propagation, plastic deformation and the formation of shear bands.

High impact toughness of CrCoNi medium-entropy alloy at liquid-helium temperature

We show that face-centered-cubic CrCoNi medium-entropy alloy sustains a Charpy V-notch impact toughness (AK) as high as 340 J at 4 K. Deformation twinning (DT) is rendered exceptionally profuse by this alloy's low stacking fault energy, combined with liquid-helium temperature and high strain rates that both incur high stresses to favor DT across a large plastic zone. This efficiently dissipates imposed mechanical energy, accumulates defects to sustain high strain hardening, and restrains strain localization events from evolving into major shear bands. Martensitic transformation to hexagonal phase is insignificant. CrCoNi is therefore a strong competitor to the best existing cryogenic alloys.

Partially-recrystallized ferrite grains and multiple plasticity enhancing mechanisms in a medium Mn steel

We report initial and deformation microstructures of a Fe-6Mn-0.15C-1.5Si-1Al (wt%) medium Mn steel having an ultrafine duplex microstructure of ferrite and austenite. The cold-rolled and annealed material shows partially recrystallized ferrite grains. The ferrite grains contain screw dislocations formed by dislocation rearrangement upon recovery of deformed martensite. The cold-rolled and annealed material shows the activation of multiple plasticity enhancing mechanisms such as deformation twinning and deformation-induced martensitic transformation. Both partially recrystallized ferrite grains and multiple plasticity enhancing mechanisms could contribute to high yield strength and high strain hardening of the investigated medium Mn steel.

High corrosion resistance and strain hardening of high Mg Al-alloy with Er and Zr by using a new reverse stabilization process

An abnormal reverse transition of intergranular corrosion from sensitization to stabilization was observed in the Al-6.4 Mg-0.43Mn-0.37Er-0.17Zr alloy at low annealing temperatures between 220 and 260 °C for the first time. The reverse stabilized alloy annealed at 240 °C for 5 h had excellent long-term corrosion resistance in the deformed state. The corresponding transmission electron microscopy analysis showed that Al3Mg2 precipitates distributed separately along grain boundary, at triple junctions and the Al6Mn phase boundary. The combination of the low temperature reverse stabilization and retarded recrystallization by addition of Er and Zr caused the high Mg aluminum alloy to exhibit high corrosion resistance and strain hardening.

Hierarchical 3D Nanolayered Duplex-Phase Zr with High Strength, Strain Hardening, and Ductility.

Nanolayered, bimetallic composites are receiving increased attention due to an exceptional combination of strength and thermal stability not possible from their coarse-layered counterparts or constituents alone. Yet, due to their 2D planar, unidirectional arrangement, they are highly anisotropic, which results in limited strain hardening and ductility. Therefore, like many high-performance, ultrastrong materials of our time, they succumb to the usual strength-ductility trade-offs. Here we present the formation of a novel hierarchical microstructure, comprised of crystals consisting of 3D nanolayered α/β-Zr networks. By direct comparison with coarse-layered material of the same chemistry, we show that the unusual hierarchical 3D structure gives rise to high strain hardening, high strength, and high ductility. Using TEM analysis and hysteresis testing, we discovered that the 3D randomly oriented biphase boundaries result in progressively dispersive rather than localized slip with increasing strain. Dislocation activity in the α-Zr lamellae transitions from single slip to multislip and eventually to multimodal slip as strain increases. The diffusive slip-promoting properties of 3D layered networks can potentially invoke simultaneous high strength, strain hardening, and ductility, and reveal a new target in the microstructural design of high performance structural materials.

Excellent combination of cryogenic-temperature strength and ductility of high-entropy-alloy-cored multi-layered sheet

We newly design a high-entropy-alloy(HEA)-cored multi-layered sheet (MLS) containing AISI 304 stainless steel (STS) surface layers (HEA/STS MLS) by using a commercial roll-bonding procedure, and property-enhancing mechanisms such as TWinning Induced Plasticity (TWIP), TRansformation Induced Plasticity (TRIP), and geometrically necessary dislocation (GND) generation were analyzed. This MLS well satisfied a rule of mixtures because the HEA and STS layers were strongly bonded to show similar deformation mechanisms of TWIP and TRIP at both room and cryogenic temperatures. The HEA layer demonstrated synergic effects of homogenization and maximization of TWIP and TRIP mechanisms with the STS layers. In particular, very high strain hardening and persistent elongation (about 50%) as well as very high tensile strength over 1.5 GPa are quite unique and interesting results because they have not been reported in the FCC-based HEA areas yet. Our findings also demonstrate that the HEA-cored MLS fabrication can be an attractive route to a new HEA design strategy for cryogenic applications.

Achieving Gradient Martensite Structure and Enhanced Mechanical Properties in a Metastable β Titanium Alloy

Gradient materials have been reported to have superior strength–ductility combinations. In this study, gradient α″ martensite were introduced along the radial direction of cylindrical Ti-10V-2Al-3Fe (Ti-1023) samples by torsional straining, which simultaneously improved strength and ductility. The torsional strain gradient produced martensite gradient with increasing density and decreasing thickness from center to surface. α″ martensite had parallel and V-shaped morphology, which not only divided coarse β grains into finer β blocks but also blocked dislocation slip. In addition, dislocation slip in the α″ martensite and β blocks led to grain refinement. The formation of geometrically necessary dislocation (GNDs) and the increasing shear stress required for martensitic transformation contributed to high strain hardening. An optimal gradient distribution exists in torsion-processed samples for the optimal mechanical properties.

Engineering Heterogeneous Multiphase Microstructure by Austenite Reverted Transformation Coupled with Ferrite Transformation

A heterogeneous multiphase medium Mn steel (Fe-10Mn-0.47C-2Al-0.7V, wt.%) consisting of prior austenite, martensite, reverted austenite, and ferrite is, for the first time, realized by both austenite reverted transformation and ferrite transformation. The phase fraction, kinetics, mechanisms, and microstructure evolution during the transformation sequence are systematically investigated. The heterogeneous multiphase medium Mn steel has great potential for high strain hardening by a continuous transformation-induced plasticity effect, grain-size gradient, and stress–strain partitioning. The present work provides a new pathway to design a heterogeneous multiphase microstructure in medium Mn steel.

High Corrosion Resistance and Strain Hardening of High Mg Al-Alloy with Er and Zr by Using a New Reverse Stabilization Process

An abnormal reverse transition of intergranular corrosion from sensitization to stabilization was observed in the Al-6.4Mg-0.43Mn0.37Er-0.17Zr alloy at low annealing temperatures between 220-260℃ for the first time. The reverse stabilized alloy annealed at 240℃ for 5h had excellent long-term corrosion resistance in the deformed state. The corresponding transmission electron microscopy analysis showed that Al3Mg2 precipitates distributed separately along grain boundary, at triple junctions and the Al6Mn phase boundary. The combination of the low temperature reverse stabilization and retarded recrystallization by addition of Er and Zr caused the high Mg aluminum alloy to exhibit high corrosion resistance and strain hardening.

Fabrication and mechanical properties of TiC reinforced CoCrFeMnNi high-entropy alloy composite by water atomization and spark plasma sintering

In this study, the TiC-reinforced CoCrFeMnNi high-entropy alloy (HEA) composite was fabricated using water atomization (WA), mechanical milling (MM), and spark plasma sintering (SPS). The microstructural evolution and mechanical properties of TiC-reinforced HEA composite are investigated using electron backscatter diffraction, transmission electron microscopy, and room temperature compression tests. The addition of 5 wt% of TiC nano-particles to CoCrFeMnNi HEA resulted in fine grain size, high yield strength, and high strain hardening. The average grain size achieved for alloys with and without TiC after sintering is 5.1 μm and 10.6 μm, respectively. The addition of TiC increases the compressive yield strength from ∼507 MPa to ∼698 MPa and compressive fracture strength from ∼1527 MPa to ∼2216 MPa, without sacrificing the ductility. The strengthening behavior of TiC-reinforced CoCrFeMnNi HEA composite is quantitatively discussed based on grain boundary strengthening, dislocation strengthening, and dispersion strengthening. The role of TiC nano-particles in the strain hardening improvement is investigated with respect to the dislocation-particle interaction and consequently increased dislocation density.

Numerical Investigation of Seismic Response of Hybrid Buckling Restrained Braced Frames

The Conventional buckling restrained braces used in concentrically braced frames are expected to yield in both tension and compression without major degradation of capacity under severe seismic ground motions. One of the weakness points of a standard buckling restrained braced frame is the low post-yield stiffness and thus large residual deformation under moderate to severe ground motions. This phenomenon can be related to the low post-yield stiffness of the core segment in comparison to its elastic stiffness. This paper investigates the application of stainless steel as the core material in a hybrid buckling restrained brace. The “hybrid” term arises from the use of more than one core component with different steel materials, including high strength high-performance steel and stainless steel (304L) with high strain hardening in the core of buckling restrained brace. Nonlinear dynamic time history analyses were conducted on a variety of diagonally braced frames with different heights, in order to compare the seismic performance of standard (non-hybrid) and hybrid buckling restrained braced frames. The results showed that the proposed hybrid buckling restrained braces reduce the inter-story and specially the residual drift demands in buckling restrained braced frames.

Enhancing strength without compromising ductility in copper by combining extrusion machining and heat treatment

It is a challenge to produce metallic materials with high strength and good ductility. Improving the strength of metallic materials usually sacrifices the ductility or work-hardening capacity. Here combining extrusion machining and heat treatment, we improve the strength of copper without losing strain hardening capacity and therefore the ductility remains. Copper was first deformed by extrusion machining at shear strain 3.1 and then annealed at 523 K for 5 min. Compared with the initial workpiece, the processed copper possesses five times higher yield strength and alike work hardening behavior. Microstructural characterizations illustrate that high strength and high strain hardening are attributed to the hierarchical microstructure that the recrystallized grains are surrounded by elongated subgrains. Finally, an analytical modeling was employed to rationalize the mechanical properties of copper processed by the proposed strategy. The theoretical results are in agreement with the experimental measurements.

Numerical Analysis of Effect of Temperature on Ball Indentation Behaviour of Armox500T and IN718

Temperature dependence of ball indentation behaviour of Armox500T and IN718 in terms of Meyer’s hardness, constraint factor (CF) and pile-up around indentation as a function of average strain at ambient temperature and elevated temperature (673 K) has been analysed using FEA. Subsequently, FE model has been validated by experimental data for IN718 and Armox500T. The value of CF for Armox500T and IN718 at RT is found to be 2.80 and 2.75, respectively. The CF value is increased to 3.66 and 3.41, respectively, at 673 K indicating the dependence of temperature. It is also observed that pile-up at a given strain for Armox500T is higher (low strain hardening) compared to IN718 (high strain hardening) under static indentation conditions. The FEA results are in good agreement with expansion cavity model and fully plastic model.

A study of the dynamic impact behaviour of IN 718 and ATI 718Plus® superalloys

ABSTRACTThe dynamic impact response of IN 718 and ATI 718Plus®, in both the solution heat treated and age-hardened conditions, were investigated at different deformation temperatures and strain rates using a direct impact Hopkinson pressure bar. Analyses of the results provide a vital but previously not reported information that the ATI 718Plus® offers a higher resistance to damage during high strain rate ballistic impact deformation compared to the most widely used Iron-nickel based superalloy, Inconel 718. ATI 718Plus® showed higher strain hardening and strain rate sensitivity, in both heat treatment conditions, than IN 718. The difference in the deformation behaviour of both alloys, in the annealed condition, is attributable to the compositional modification in ATI 718Plus® which has been reported to lower its stacking fault energy and increases the tendency for deformation twinning. However, in the age-hardened condition, the difference is believed to be related to the disparity in the operative strengthening mechanism, of the precipitates present in both alloys. Furthermore, a higher susceptibility to strain location and the formation of adiabatic shear band, in aged IN 718, is attributable to the stronger temperature-softening characteristics observed in the alloy and to the limited strain hardening tendency under dynamic impact loading.

Tensile deformation and fracture mechanisms of Cu/Nb nanolaminates studied by in situ TEM mechanical tests

The mechanisms of deformation and failure of Cu/Nb nanolaminates manufactured by accumulated roll bonding were analysed using in-situ TEM mechanical tests. Dog-bone small-scale tensile specimens were prepared using a FIB-based technique with the layers parallel and perpendicular to the loading axis. Load, deformation and TEM micrographs were recorded during the in-situ TEM mechanical tests. The specimens deformed parallel to the layers presented very high strain hardening and ductility while those deformed in the perpendicular orientation were more brittle. These results were rationalized in terms of the deformation and fracture mechanisms observed during the tests.

Determining the strain to fracture for simple shear for a wide range of sheet metals

There is a particularly high degree of experimental uncertainty associated with the determination of the strain to fracture for simple shear. In most shear specimens, different stress states coexist and it is usually impossible to prove experimentally that fracture indeed initiated in the sheared zones. Peirs’ simple shear specimen is analyzed in detail for nine generic materials covering different combinations from low to high strain hardening and from low ductility to high ductility. It is found that the validity of the shear fracture strain measurement depends on the strain hardening capability and the ductility of the material being tested, with a higher likelihood of validity for low hardening and low ductility materials. An extensive parametric study evaluating more than 600 distinct specimen geometries is performed in an attempt to identify a universal specimen geometry. It turns out that no single geometry exists which provides reliable measurements for all types of materials. While a single geometry can be found for different hardening behaviors, it seems to be necessary to use several distinct geometries to cover all levels of ductility. Since the ductility is usually not known prior to testing, we recommend testing three different types of specimen per material, reporting the highest measured strain among all specimens as strain to fracture for simple shear. Experiments are performed on specimens extracted from aluminum AA2024-T351, as well as DP800 and DP980 dual phase steels to demonstrate the validity of the proposed experimental methodology for identifying the strain to fracture for simple shear.